A previously isolated recombinant clone expressing a 59 kDa antigen (p59) that reacts with monospecific and monoclonal antibodies directed against type 2 fimbriae from Actinomyces viscosus T14V has been further characterized. The fimA gene which encodes p59 hybridized strongly with a gene encoding a subunit of Actinomyces naeslundii type 2 fimbriae and more weakly with an A. viscosus T14V gene encoding a subunit of type 1 fimbriae, which suggests that these three genes may be derived from a common ancestral gene. To define fimA in detail, a number of subclones have been isolated and sequenced; so far about 70% of the sequence of one strand has been obtained. To investigate whether the cloned subunit expresses the lectin activity that is associated with intact fimbriae and which mediates coaggregation with certain oral streptococci, p59 was purified to homogeneity in three steps and injected into a rabbit to raise antibodies. During this purification of two antigen- containing peaks were observed both after DEAE-chromatography and gel filtration in the presence of Triton X-100 which differed about 2 kDa in size. It appears likely that the smaller form is due to the removal of a hydrophobic leader by the Escherichia coli recombinant cells. In order to (re)introduce cloned genes into oral actinomyces, a number of recently isolated bacteriophages have been examined to evaluate whether they can be utilized to develop a transformation system for A. viscosus. One group of small, short-tailed, phages contained linear, non-permuted, double-stranded DNA, 18-19 kb in size. Two other phages had double-stranded DNA of about 40 and 80 kb, respectively. In another study, the receptor for the small phages on the surface of A. viscosus has been examined to see if it is related to one of the coaggregation mediators. The results obtained with various phage-resistant mutants indicate that the receptor for these bacteriophages is identical, or related, to the carbohydrate that interacts with streptococci belonging to coaggregation groups 1 and 2.